Carbon fibers are produced from a wide variety of precursors, the most common being polyacrylonitrile, pitch, pitch mesophase and benzene decomposition. These are discussed generally in J. D. Donnet and R. C. Bansal, "Carbon Fibers", Marcel Dekker, New York, 1984. The production of such fibers requires treating the fiber precursor to elevated temperatures in excess of 2000.degree. C. In the heat treating process of such polymer fibers, it is well known that partially or fully graphitized structures may be developed, but the degree of disorder, the orientation of the basal plane, and the extent of impurity depend on a combination of the heat treatment temperature, tension of the fiber during the heating process, and the structure of the precursor. Frequently, heat treatment in such processes is conducted at temperatures as high as 3200.degree. C. before well-developed graphitic structures are encountered and conductivity approaches that of a single crystal graphite.
It is recognized that carbon fibers have very desirable thermal and mechanical stability, and can be fabricated into continuous filaments and/or films. Further, it is also recognized that such properties are desirable in a superconductor. To this end, carbon fibers coated by chemical vapor deposition with niobium carbonitride (NbCN) films are known and have been studied extensively because the fiber substrate allows processing while the NbCN has desirable superconducting properties.
U.S. Pat. No. 4,585,696, U.S. Pat. No. 4,299,861, U.S. Pat. No. 4,657,776, and U.S. Pat. No. 4,581,289 disclose superconducting fibers consisting of carbon fiber bases each covered with a transition metal carbide superconducting layer.
In the '696 patent, this layer may consist of niobium carbonitride or a niobium oxycarbonitride. The niobium is simultaneously deposited through chemical vapor deposition and carbonitrided in a carbon and nitrogen containing atmosphere. This superconducting layer is then covered with a layer of high purity copper or aluminum.
Similarly, the superconducting layer in the 861 patent also consists of a niobium carbonitride layer. The layer is formed by first coating the carbon fiber with niobium by chemical vapor deposition and then heating it in a carbon and nitrogen atmosphere. The '776 and '289 patents relate to processes for chemical vapor deposition of niobium carbonitride on a carbon fiber carrier. This superconducting layer is then covered with high purity copper.
U.S. Pat. No. 4,649,038 of common inventorship and assignment herewith, pertains to a conductive polymer prepared from a novel polycyanogen, to fibers made therefrom, and to the method of preparation of these polymers and fibers.